Centrifugal compressor

ABSTRACT

A centrifugal compressor includes one or more impellers, a casing which covers the one or more impellers, and a silencer. The casing includes a radial passage which guides a gas flowing out from a final stage impeller to a radially outer side, and an outlet scroll passage which communicates the radial passage with an outlet. The silencer includes a perforated plate in which a plurality of acoustic holes are formed, and a space forming body which forms an acoustic space connected to the plurality of acoustic holes. A first surface of the silencer opposite to a surface of the perforated plate defining the acoustic space forms at least part of an inner surface of the casing which defines the outlet scroll passage.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a centrifugal compressor.

Priority is claimed on Japanese Patent Application No. 2018-246653,filed on Dec. 28, 2018, the content of which is incorporated herein byreference.

Description of Related Art

A centrifugal compressor includes one or more impellers which send gasoutward in a radial direction by rotation about an axis, and a casingwhich covers the one or more impellers.

In United States Patent Application, Publication No. 2002/0079158, acentrifugal compressor including a silencer to reduce noise emitted froma centrifugal compressor is disclosed. The centrifugal compressor ofPatent Document 1 includes a plurality of impellers which send gasoutward in the radial direction by rotation about an axis, a casingwhich covers the plurality of impellers, and the above-describedsilencer. The plurality of impellers are arranged in an axial directionof the impellers. An inlet which introduces gas into the inside, anoutlet which discharges the gas, and a passage which guides the gasflowing in from the inlet to the outlet through the plurality ofimpellers are formed in the casing. The passage has a radial passage andan outlet scroll passage. The radial passage is a passage which guidesthe gas flowing out from the final stage impeller among the plurality ofimpellers outward in the radial direction. The outlet scroll passage isa passage which extends in a circumferential direction around the axisand allows communication between a radially outer portion of the radialpassage and the outlet.

The silencer includes a perforated plate and a space forming body. Theperforated plate has a first surface and a second surface, and aplurality of acoustic holes which penetrate from the first surface tothe second surface opposite to the first surface. The space forming bodyforms an acoustic space on the second surface side of the perforatedplate. The silencer is disposed so that the first surface of theperforated plate forms part of an inner surface of the casing whichdefines the radial passage.

SUMMARY OF THE INVENTION

In this centrifugal compressor, it is required to further reduce noise,that is, to further enhance sound absorption effects.

An object of the present invention is to provide a centrifugalcompressor which is able to enhance a sound absorption effect.

According to a first aspect of the present invention, there is provideda centrifugal compressor including one or more impellers configured torotate about an axis and to send a gas to a radially outer side of theaxis and to guide the gas from an inlet to an inside to discharge thegas from an outlet, a casing configured to cover the one or moreimpellers, and a silencer. The casing has an outlet scroll passage whichextends in a circumferential direction of the axis and communicates withthe outlet. The silencer includes a perforated plate in which a firstsurface and a second surface opposite to the first surface and aplurality of acoustic holes penetrating from the first surface to thesecond surface are formed, and a space forming body configured to forman acoustic space connected to the plurality of acoustic holes on asecond surface side of the perforated plate. The first surface of thesilencer forms part of an inner surface of the casing which defines theoutlet scroll passage.

The centrifugal compressor of the aspect includes a silencer which usesthe principle of a Helmholtz resonator. Tests were performed on arelationship between the sound absorption coefficient of the silencerusing the principle of a Helmholtz resonator and the velocity of the gasflowing through the passage. As a result, it was found that the soundabsorption coefficient of the silencer which faces a passage portionincreases as the velocity of the gas flowing through the passage portiondecreases.

The velocity of the gas flowing through the outlet scroll passage islower than that of the gas flowing through the radial passage. Thesilencer in this aspect is disposed such that the first surface of theperforated plate forms at least part of the inner surface of the casingwhich defines the outlet scroll passage. Therefore, in this aspect, thesound absorption coefficient can also be increased by a silencer whichfaces a portion of the radial passage in which a flow rate of the gas isfaster than that in the outlet scroll passage, and thereby the soundabsorption effect can be enhanced.

Here, in the centrifugal compressor of the first aspect, the firstsurface of the silencer may form only part of the inner surface of theoutlet scroll passage.

Further, in the centrifugal compressor of the first aspect, a firstsilencer and a second silencer may be provided as the silencers. In thiscase, the casing may have a radial passage which guides the gas flowingout from a final stage impeller of the one or more impellers to theradially outer side and communicates with the outlet scroll passage. Thefirst surface of the first silencer may form part of the inner surfaceof the outlet scroll passage, and the first surface of the secondsilencer may form part of an inner surface of the casing which definesthe radial passage.

Since the centrifugal compressor of the aspect includes the firstsilencer and the second silencer, it is possible to increase a soundabsorption amount compared with a case in which only the first silenceris provided.

In the centrifugal compressor according to the aspect having the secondsilencer, the first surface of the second silencer may not form an innersurface in an inner region including an end radially inward with respectto the axis but may form at least part of the inner surface in an outerregion on the radially outer side from the inner region in the radialpassage.

In the radial passage, the velocity of the gas is lower in the outerregion than in the inner region. Thus, in this aspect, the soundabsorption coefficient of the second silencer can be increased.

Further, in the centrifugal compressor according to any one of theaspects having the second silencer, a short distance silencer and a longdistance silencer may be provided as the second silencers. In this case,the short distance silencer may be disposed at a position closer to thefinal stage impeller than the long distance silencer is in a radialdirection of the axis, and inner diameters of the plurality of acousticholes of the short distance silencer may be smaller than those of theplurality of acoustic holes of the long distance silencer.

Tests were performed regarding the relationship between the soundabsorption coefficient of the silencer using the principle of aHelmholtz resonator, the velocity of the gas flowing through thepassage, and the inner diameter of the acoustic holes formed in theperforated plate. As a result of the test, it has been found that thesound absorption coefficient increases when the inner diameter of theacoustic holes is reduced. In other words, it has been found that evenwhen the velocity of the gas flowing through the passage is high, adecrease in the sound absorption coefficient can be curbed by reducingthe inner diameter of the acoustic holes. In the radial passage, thevelocity of the gas is higher at a position close to the final stageimpeller in the radial direction than at a position far from the finalstage impeller.

In this aspect, since the inner diameters of the plurality of acousticholes of the short distance silencer is smaller than those of theplurality of acoustic holes of the long distance silencer, even when theshort distance silencer is disposed in a region in which the velocity ofthe gas is high, a decrease in the sound absorption coefficient of theshort distance silencer can be curbed.

To solve the above problem, according to a second aspect of the presentinvention, there is provided a centrifugal compressor including one ormore impellers configured to rotate about an axis and to send a gas to aradially outer side of the axis, a casing configured to cover the one ormore impellers and to guide the gas from an inlet to an inside todischarge the gas from an outlet, and a short distance silencer and along distance silencer as silencers. The casing has an outlet passagewhich guides the gas flowing out from a final stage impeller of the oneor more impellers to the outlet. Each of the short distance silencer andthe long distance silencer includes a perforated plate in which a firstsurface and a second surface opposite to the first surface and aplurality of acoustic holes penetrating from the first surface to thesecond surface are formed, and a space forming body configured to forman acoustic space connected to the plurality of acoustic holes on asecond surface side of the perforated plate. Both the first surface ofthe short distance silencer and the first surface of the long distancesilencer form part of an inner surface of the casing which defines theoutlet passage. A distance along the outlet passage from the final stageimpeller to the short distance silencer is shorter than that along theoutlet passage from the final stage impeller to the long distancesilencer. Inner diameters of the plurality of acoustic holes of theshort distance silencer are smaller than those of the plurality ofacoustic holes of the long distance silencer.

Also in this aspect, since the inner diameters of the plurality ofacoustic holes of the short distance silencer are smaller than those ofthe plurality of acoustic holes of the long distance silencer, even whenthe short distance silencer is disposed in a region in which thevelocity of the gas is high, a decrease in the sound absorptioncoefficient of the short distance silencer can be curbed.

In the centrifugal compressor according to any one of the aspects, atleast part of the space forming body of the silencer may be formed ofthe same material as that of the casing and may be integrally formedwith the casing.

In this aspect, as a portion of the silencer, a portion except part ofthe space forming body formed integrally with the casing can be easilyinstalled in and separated from the casing.

Here, in the centrifugal compressor of the aspect, the perforated platemay be formed of a material having a linear expansion coefficient largerthan that of the casing.

In this aspect, due to a difference in the linear expansion coefficientbetween at least part of the space forming body and the casing and theperforated plate, a gap is formed between the perforated plate and atleast part of the space forming body, and a gap is formed between theperforated plate and the casing according to the situation of thecentrifugal compressor.

When the gas compressed by the centrifugal compressor contains acomponent which becomes a liquid at room temperature, and thecentrifugal compressor is cooled and the centrifugal compressor and thegas therein reach room temperature, some of the gas changes to a liquid.The liquid may remain in the acoustic space of the silencer. Forexample, in a starting process of the centrifugal compressor, all theliquid in the acoustic space may not be vaporized, and some of theliquid may remain as a liquid. In this case, a problem that the silencercannot absorb sound in a target frequency range may occur.

As described above, since a gap is formed between a plurality of partsconstituting the silencer, the silencer in this aspect can discharge aliquid from the gap even when a liquid remains in the acoustic space.Thus, in this aspect, a problem in that the silencer cannot absorb thesound in the target frequency range can be avoided, and reduction in thesound absorption effect can be curbed.

In the centrifugal compressor according to any one of the aspects, thesilencer may have a drain hole which discharges a liquid in the acousticspace outside of the acoustic space.

Since the silencer in this aspect has the drain hole, even when a liquidremains in the acoustic space, the liquid can be discharged outside theacoustic space from the drain hole. Accordingly, in this aspect, theproblem that the silencer cannot absorb the sound in the targetfrequency range can be avoided, and the reduction of the soundabsorption effect can be curbed.

To solve the above problem, according to a third aspect of the presentinvention, there is provided a centrifugal compressor including one ormore impellers configured to rotate about an axis and to send a gas to aradially outer side of the axis, a casing configured to cover the one ormore impellers and to guide the gas from an inlet to an inside todischarge the gas from an outlet, and a silencer. The casing has apassage which guides the gas flowing in from the inlet to the outletthrough the one or more impellers. The silencer includes a perforatedplate in which a first surface and a second surface opposite to thefirst surface and a plurality of acoustic holes penetrating from thefirst surface to the second surface are formed, a space forming bodyconfigured to form an acoustic space connected to the plurality ofacoustic holes on a second surface side of the perforated plate, and adrain hole configured to discharge a liquid in the acoustic spaceoutside of the acoustic space. The first surface of the silencer formspart of an inner surface of the passage.

Since the silencer in this aspect has the drain hole, even when a liquidremains in the acoustic space, the liquid can be discharged from thedrain hole to the outside of the acoustic space. Therefore, in thisaspect, the problem that the silencer cannot absorb the sound in thetarget frequency range can be avoided, and the reduction of the soundabsorption effect can be curbed.

In the centrifugal compressor according to any one of the aspects havingthe drain hole, the drain hole may have a first opening which opens inan inner surface of the space forming body which faces the acousticspace, and a second opening which opens in an outer surface of thecasing.

In the centrifugal compressor according to the aspect having the secondopening which opens in the outer surface of the casing, an opening areaof the first opening may be 20% or less of a total opening area of allthe acoustic holes in a portion of the perforated plate which definesone acoustic space.

In the centrifugal compressor according to any one of the aspects havingthe second opening which opens in the outer surface of the casing, thesilencer may have a valve capable of partitioning a space in the drainhole from an outer space of the casing.

Further, in the centrifugal compressor according to any one of theaspects having the second opening which opens in the outer surface ofthe casing, a moisture absorbent may be disposed in the drain hole.

In the centrifugal compressor according to any one of the aspects havingthe drain hole, the drain hole may have a first opening which opens inthe first surface of the perforated plate, and a second opening whichopens in the second surface of the perforated plate, and a moistureabsorbent may be disposed in the drain hole.

To solve the above problem, according to a fourth aspect of the presentinvention, there is provided a centrifugal compressor including one ormore impellers configured to rotate about an axis and to send a gas to aradially outer side of the axis, a casing configured to cover the one ormore impellers and to guide the gas from an inlet to an inside todischarge the gas from an outlet, and a silencer. The casing has apassage configured to guide the gas flowing in from the inlet to theoutlet through the one or more impellers. The silencer includes aperforated plate in which a first surface and a second surface oppositeto the first surface and a plurality of acoustic holes penetrating fromthe first surface to the second surface are formed, and a space formingbody configured to form an acoustic space connected to the plurality ofacoustic holes on a second surface side of the perforated plate. Thefirst surface of the silencer forms part of an inner surface of thecasing which defines the passage. At least part of the space formingbody is formed of the same material as that of the casing and integrallyformed with the casing. The perforated plate is formed of a materialhaving a linear expansion coefficient larger than that of the casing.

Also in the silencer in this aspect, since a gap is formed between aplurality of parts constituting the silencer, the liquid can bedischarged from the gap even when a liquid remains in the acousticspace. Therefore, also in this aspect, the problem that the silencercannot absorb the sound in the target frequency range can be avoided,and the reduction of the sound absorption effect can be curbed.

In the centrifugal compressor according to any one of the aspects, innerdiameters of the plurality of acoustic holes may be 0.1 mm or more to3.0 mm or less.

Further, in the centrifugal compressor according to any one of theaspects, inner diameters of the plurality of acoustic holes may be 0.2mm or more to 1.0 mm or less.

In one aspect of the present invention, it is possible to increase asound absorption effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a centrifugal compressor accordingto a first embodiment.

FIG. 2 is a cross-sectional view of a main part of the centrifugalcompressor according to the first embodiment.

FIG. 3 is a graph showing a relationship between a sound absorptioncoefficient of a silencer and a velocity of a gas flowing through apassage.

FIG. 4 is a cross-sectional view of a main part of a centrifugalcompressor according to a first modified example of the firstembodiment.

FIG. 5 is a cross-sectional view of a main part of a centrifugalcompressor according to a second modified example of the firstembodiment.

FIG. 6 is a cross-sectional view of a main part of a centrifugalcompressor according to a second embodiment.

FIG. 7 is a cross-sectional view of a main part of a centrifugalcompressor according to a third embodiment.

FIG. 8 is a cross-sectional view of a main part of a centrifugalcompressor according to a fourth embodiment.

FIG. 9 is a cross-sectional view of a main part of a centrifugalcompressor according to a fifth embodiment.

FIG. 10 is a cross-sectional view of a main part of the centrifugalcompressor in a state in which the centrifugal compressor according tothe fifth embodiment is cooled.

FIG. 11 is a cross-sectional view of a main part of a centrifugalcompressor according to a sixth embodiment.

FIG. 12 is a cross-sectional view of a main part of a centrifugalcompressor according to a seventh embodiment.

FIG. 13 is a cross-sectional view around a silencer of the centrifugalcompressor according to the seventh embodiment.

FIG. 14 is a cross-sectional view around a silencer of a centrifugalcompressor according to an eighth embodiment.

FIG. 15 is a cross-sectional view around a silencer of a centrifugalcompressor according to a ninth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, various embodiments of a centrifugal compressor accordingto the present invention will be described with reference to thedrawings.

First Embodiment

A centrifugal compressor according to a first embodiment will bedescribed with reference to FIGS. 1 to 3.

The centrifugal compressor according to this embodiment is a multistagecentrifugal compressor. As shown in FIG. 1, the centrifugal compressoraccording to this embodiment includes a rotary shaft 10, bearings 15 aand 15 b, a plurality of impellers 11, a casing 20, and a silencer 30A.The rotary shaft 10 has a cylindrical shape and rotates around an axisAr. The bearings 15 a and 15 b rotatably support the rotary shaft 10.The plurality of impellers 11 are installed on the rotary shaft 10 androtate around the axis Ar together with the rotary shaft 10. The casing20 surrounds the impellers 11. The silencer 30A is provided in thecasing 20.

Here, a direction in which the axis Ar extends is referred to as anaxial direction Da. The bearing 15 a side in the axial direction Da isreferred to as the first side Da1. The bearing 15 b side is referred toas the second side Da2. Further, a radial direction with respect to theaxis Ar is referred to as a radial direction Dr. The side closer to theaxis Ar in the radial direction Dr is referred to as the radially innerside Dri. The side away from the axis Ar in the radial direction Dr isreferred to as the radially outer side Dro. A circumferential directionaround the axis Ar is referred to as a circumferential direction Dc.

The rotary shaft 10 is provided to pass through the casing 20 in theaxial direction Da. The centrifugal compressor according to thisembodiment includes a thrust bearing 15 a and a journal bearing 15 b asthe bearings 15 a and 15 b. The thrust bearing 15 a is provided at anend of the casing 20 on the first side Da1 and supports an end of therotary shaft 10 on the first side Da1. The journal bearing 15 b isprovided at an end of the casing 20 on the second side Da2 and supportsan end of the rotary shaft 10 on the second side Da2.

The plurality of impellers 11 are arranged in the axial direction Da tobe spaced apart from each other in the axial direction Da. Anin-impeller flow path 12 through which a gas flows is formed in each ofthe impellers 11. The in-impeller flow path 12 has an impeller inlet 13and an impeller outlet 14. The impeller inlet 13 is open from the insideof the in-impeller flow path 12 toward the first side Da1 in the axialdirection Da. The impeller outlet 14 is open from the inside of thein-impeller flow path 12 toward the radially outer side Dro. Thein-impeller flow path 12 is formed to be gradually directed to theradially outer side Dro as it goes from the first side Da1 to the secondside Da2 in the axial direction Da. A cross-sectional area of thein-impeller flow path 12 gradually reduces from the impeller inlet 13toward the impeller outlet 14. Hereinafter, among the plurality ofimpellers 11, the impeller 11 located furthest toward the first side Da1in the axial direction Da is referred to as an initial stage impeller 11a, and the impeller 11 located on the most second side Da2 in the axialdirection Da is referred to as a final stage impeller 11 b.

The casing 20 has an inlet 20 i which guides a gas into the casing 20,an outlet 20 o which discharges the gas, and a passage 20 p. The inlet20 i is formed on the first side Da1 with respect to the plurality ofimpellers 11. The outlet 200 o is formed on the second side Da2 withrespect to the plurality of impellers 11. The passage 20 p guides thegas flowing in from the inlet 20 i to the outlet 20 o via the pluralityof impellers 11.

The casing 20 has an inlet casing 21, an intermediate casing 23, and anoutlet casing 25. The inlet casing 21 has an inlet 20 i and an inletpassage 21 p. The inlet passage 21 p guides the gas flowing in from theinlet 20 i to the impeller inlet 13 of the initial stage impeller 11 a.The intermediate casing 23 partitions the two impellers 11 adjacent toeach other in the axial direction Da. The intermediate casing 23 isdisposed between the inlet casing 21 and the outlet casing 25 in theaxial direction Da. The intermediate casing 23 has a plurality ofintermediate passages 23 p. The intermediate passage 23 p guides the gasflowing out from the impeller outlet 14 of the impeller 11 on the firstside Da1 to the impeller inlet 13 of the impeller 11 on the second sideDa2, among the two impellers 11 adjacent to each other in the axialdirection Da.

The intermediate passage 23 p has a diffuser passage 23 pa, a turningpassage 23 pb, and a return passage 23 pc. The diffuser passage 23 paguides the gas flowing out from the impeller outlet 14 of the impeller11 on the first side Da1, among the two impellers 11 adjacent to eachother in the axial direction Da, to the radially outer side Dro. Thatis, the diffuser passage 23 pa is a passage which extends in the radialdirection Dr. The turning passage 23 pb is connected to an end of thediffuser passage 23 pa on the radially outer side Dro. The turningpassage 23 pb is a passage which changes a flow of the gas flowing infrom the diffuser passage 23 pa from the radially outer side Dro to theradially inner side Dri. The return passage 23 pc is connected to theturning passage 23 pb. The return passage 23 pc guides the gas flowingin from the turning passage 23 pb to the impeller inlet 13 of theimpeller 11 on the second side Da2, among the two impellers 11 adjacentto each other in the axial direction Da.

As described above, the intermediate casing 23 has the intermediatepassage 23 p. In addition, the intermediate casing 23 forms part of aradial passage 26 as shown in FIG. 2. The radial passage 26 is a passagewhich guides the gas flowing out from the impeller outlet 14 of thefinal stage impeller 11 b to the radially outer side Dro.

As shown in FIGS. 1 and 2, the outlet casing 25 has an outlet 20 o andan outlet scroll passage 27. The outlet casing 25 also forms part of theabove-described radial passage 26. The radial passage 26 extends fromthe impeller outlet 14 of the final stage impeller 11 b toward theradially outer side Dro and also expands in the circumferentialdirection Dc. The intermediate casing 23 forms a first side innersurface 26 a among inner surfaces forming the radial passage 26. Theoutlet casing 25 forms a second side inner surface 26 b among the innersurfaces forming the radial passage 26. Both the first side innersurface 26 a and the second side inner surface 26 b expand in the radialdirection Dr and the circumferential direction Dc. The second side innersurface 26 b is located closer to the second side Da2 than the firstside inner surface 26 a. The second side inner surface 26 b faces thefirst side Da1. The first side inner surface 26 a faces the second sideDa2.

The outlet scroll passage 27 extends in the circumferential direction Dcand allows communication between a portion of the radial passage 26 onthe radially outer side Dro and the outlet 20 o. An inner surface of theoutlet scroll passage 27 which extends in the circumferential directionDc has an external inner surface 27 o, an internal inner surface 27 i, afirst side inner surface 27 a, and a second side inner surface 27 b. Theexternal inner surface 270 and the internal inner surface 27 i expand inthe circumferential direction Dc and the axial direction Da. Theexternal inner surface 27 o is located closer to the radially outer sideDro than the internal inner surface 27 i. The external inner surface 27o faces the radially inner side Dri. The internal inner surface 27 ifaces the radially outer side Dro. The first side inner surface 27 a andthe second side inner surface 27 b expand in the circumferentialdirection Dc and the radial direction Dr. The second side inner surface27 b is located closer to the second side Da2 than the first side innersurface 27 a. The second side inner surface 27 b faces the first sideDa1. The first side inner surface 27 a faces the second side Da2.

The radial passage 26 and the outlet scroll passage 27 constitute anoutlet passage 25 p. The inlet passage 21 p, the plurality ofintermediate passages 23 p, and the outlet passage 25 p constitute thepassage 20 p.

The silencer 30A includes a perforated plate 31 and a space forming body35. A first surface 32 and a second surface 33 opposite to the firstsurface 32, and a plurality of acoustic holes 34 are formed in theperforated plate 31. The first surface 32 and the second surface 33constitute both surfaces of the perforated plate 31. The plurality ofacoustic holes 34 penetrate from the first surface 32 to the secondsurface 33. Inner diameters of the plurality of acoustic holes 34 arepreferably 0.1 mm or more to 3.0 mm or less. The inner diameters of theplurality of acoustic holes 34 are, for example, 1.0 mm.

The space forming body 35 is a member which forms a space on the secondsurface 33 side of the perforated plate 31. The space forming body 35includes an outer frame 36 which defines an outer edge of a spacebetween the outer frame 36 and the perforated plate 31, and a partitionwall 39 which partitions the space into a plurality of acoustic spacesS. Each of the acoustic spaces S communicates with one or more of theplurality of acoustic holes 34. Therefore, the silencer 30A of thisembodiment is a silencer which uses the principle of a Helmholtzresonator.

The silencer 30A is disposed so that the first surface 32 of theperforated plate 31 forms part of the external inner surface 27 o amongthe inner surfaces which define the outlet scroll passage 27 in thecasing 20.

The inventor performed a test regarding a relationship between a soundabsorption coefficient of the silencer using the principle of aHelmholtz resonator and a velocity of the gas flowing through thepassage. Test results will be described with reference to a graph shownin FIG. 3. A horizontal axis in the graph shown in FIG. 3 indicates thevelocity (Mach number) of the gas flowing through a passage portionwhich is a portion of the passage through which the gas flows and whichfaces the perforated plate of the silencer. A vertical axis in the graphshown in FIG. 3 is the sound absorption coefficient of the silencer.Inner diameters of the plurality of acoustic holes in the silencer usedin the test are 1.0 mm, similar to the inner diameters of the pluralityof acoustic holes 34 in this embodiment.

As shown in the graph of FIG. 3, when the velocity of the gas flowingthrough the passage portion is Mach number 0.6, the sound absorptioncoefficient of the silencer facing the passage portion is about 0.2.When the velocity of the gas flowing through the passage portion is Machnumber 0.4, the sound absorption coefficient of the silencer facing thepassage portion is about 0.4. When the velocity of the gas flowingthrough the passage portion is Mach number 0.0, the sound absorptioncoefficient of the silencer facing the passage portion is about 0.6.Therefore, as the velocity of the gas flowing through the passageportion becomes lower, the sound absorption coefficient of the silencerfacing the passage portion becomes greater.

As described above, the radial passage 26 extends from the impelleroutlet 14 of the final stage impeller 11 b toward the radially outerside Dro and also expands in the circumferential direction Dc. Thus, across-sectional area of the radial passage 26 gradually increases towardthe radially outer side Dro. Therefore, the velocity of the gas flowingthrough the radial passage 26 gradually decreases as it flows toward theradially outer side Dro. The velocity of the gas flowing through theoutlet scroll passage 27 is lower than that of the gas flowing throughthe radial passage 26.

Since the silencer 30A faces part of the outlet scroll passage 27, thesound absorption coefficient is higher than that of the silencer facingpart of the radial passage 26 in which a flow rate of the gas is fasterthan that in the outlet scroll passage 27. Therefore, the soundabsorption coefficient can be enhanced in the silencer 30A.

When the silencer is disposed to face the passage, a pressure loss ofthe gas occurs due to the presence of the plurality of acoustic holes 34of the silencer. The pressure loss is proportional to the square of thevelocity of the gas. Since the silencer 30A faces part of the outletscroll passage 27, the pressure loss of the gas can be made smaller thanthat in a silencer which faces part of the radial passage 26 in whichthe flow rate of the gas is faster than that in the outlet scrollpassage 27.

In the centrifugal compressor according to this embodiment and acentrifugal compressor having the same configuration as that of thecentrifugal compressor according to this embodiment except for thesilencer 30A, the velocity of the gas at the impeller outlet 14 of thefinal stage impeller 11 b is, for example, Mach number 0.6 to 0.8. Inother words, the velocity of the gas at the impeller outlet 14 of thefinal stage impeller 11 b is the velocity of the gas at the end of theradial passage 26 on the radially inner side Dri. The velocity of thegas flowing through the radial passage 26 gradually decreases as itflows to the radially outer side Dro due to the above-described reason.Additionally, the velocity of the gas in the outlet scroll passage 27 isMach number 0.4 or less. Therefore, the sound absorption coefficient ofthe silencer 30A which faces part of the outlet scroll passage 27becomes about 0.4 or more.

Incidentally, as a result of the test, it was found that when the innerdiameters of the acoustic holes 34 are reduced, a sound absorptioncoefficient curve is shifted to the right as shown by a broken line inFIG. 3. That is, it was found that, when the sound absorptioncoefficient is the same between a case in which the inner diameters ofthe acoustic holes 34 are large and a case in which the inner diametersof the acoustic holes 34 are small, the velocity of the gas is higher inthe case in which the inner diameters of the acoustic holes 34 aresmall. In other words, it was found that, when the velocity of the gasis the same between the case in which the inner diameters of theacoustic holes 34 are large and the case in which the inner diameters ofthe acoustic holes 34 are small, the acoustic holes 34 having smallerinner diameters has a higher sound absorption coefficient. Therefore,the sound absorption coefficient can be increased by reducing the innerdiameters of the acoustic holes 34. This is considered to be because,when the inner diameters of the acoustic holes 34 are reduced, thepressure loss of the gas passing through the acoustic holes 34increases, and the influence of the decrease in the sound absorptioncoefficient due to the velocity of the gas passing through the passageportion is reduced. Therefore, in this embodiment, the inner diameter ofthe acoustic hole 34 is set to 1.0 mm which is a minimum inner diameterthat can be drilled with a drill, or an inner diameter close to theminimum inner diameter.

Hole machining can be performed using a laser as well as drilling. Whenthe acoustic holes 34 are machined with a laser, the acoustic holes 34having an inner diameter of 1.0 mm or less can be formed. Therefore,considering the case in which the acoustic holes 34 are machined with alaser, the inner diameter of the acoustic hole 34 is preferably 0.2 mmor more to 1.0 mm or less.

[Modified Example of First Embodiment]

Various modified examples of the centrifugal compressor in the firstembodiment will be described with reference to FIGS. 4 and 5. In variousembodiments and various modified examples of the centrifugal compressorwhich will be described below, the arrangement or configuration of thesilencer is different, but the configuration is otherwise the same asthat of the centrifugal compressor according to the first embodiment.Therefore, in the following, the arrangement or configuration of thesilencer will be mainly described in various embodiments and variousmodified examples.

In the silencer 30A according to the first embodiment, the first surface32 of the perforated plate 31 is disposed to form part of the externalinner surface 27 o among the inner surfaces defining the outlet scrollpassage 27 in the casing 20. As shown in FIG. 4, a silencer 30Aa in afirst modified example of the first embodiment has the sameconfiguration as that of the silencer 30A in the first embodiment.However, in the silencer 30Aa, the first surface 32 of the perforatedplate 31 is disposed to form part of the second side inner surface 27 bamong the inner surfaces defining the outlet scroll passage 27 in theoutlet casing 25.

As shown in FIG. 5, a silencer 30Ab in a second modified example of thefirst embodiment has the same configuration as the silencer 30A in thefirst embodiment. However, the silencer 30Ab is disposed so that thefirst surface 32 of the perforated plate 31 forms part of the internalinner surface 27 i among the inner surfaces which define the outletscroll passage 27 in the outlet casing 25.

In this way, as in the first embodiment, the silencer 30Aa of the firstmodified example and the silencer 30Ab of the second modified exampleare also disposed so that the first surface 32 of the perforated plate31 forms part of the inner surface which defines the outlet scrollpassage 27 in the outlet casing 25. Therefore, as in the firstembodiment, the silencer 30Aa of the first modified example and thesilencer 30Ab of the second modified example can also increase the soundabsorption coefficient and can reduce the pressure loss of the gas.

As described above, the silencer 30A may be disposed so that the firstsurface 32 of the perforated plate 31 forms part of one of the innersurfaces which define the outlet scroll passage 27 in the outlet casing25. Therefore, the silencer may be disposed so that the first surface 32of the perforated plate 31 forms part of the first side inner surface 27a among the inner surfaces which define the outlet scroll passage 27 inthe casing 20.

Second Embodiment

A centrifugal compressor according to a second embodiment will bedescribed with reference to FIG. 6.

The centrifugal compressor according to this embodiment includes a firstsilencer 30B1 and a second silencer 30B2. As in the silencer 30Aaccording to the first embodiment, each of the first silencer 30B1 andthe second silencer 30B2 includes the perforated plate 31 in which theplurality of acoustic holes 34 are formed, and the space forming body 35which forms a plurality of acoustic spaces S.

The first silencer 30B1 is disposed so that the first surface 32 of thefirst silencer 30B 1 forms part of the external inner surface 27 o ofthe outlet scroll passage 27, as in the silencer 30A in the firstembodiment. As described in the modified example of the firstembodiment, the first silencer 30B1 may be disposed so that the firstsurface 32 of the perforated plate 31 forms part of one of the innersurfaces which define the outlet scroll passage 27.

The second silencer 30B2 is disposed so that the first surface 32 of thesecond silencer 30B2 forms part of the first side inner surface 26 a ofthe radial passage 26. The second silencer 30B2 only needs to bedisposed so that the first surface 32 of the perforated plate 31 formspart of the inner surface of the radial passage 26. For example, thesecond silencer 30B2 may be disposed to form part of the second sideinner surface 26 b of the radial passage 26.

In the of the radial passage 26, the first surface 32 of the secondsilencer 30B2 does not form the inner surface in an inner regionincluding an end on the radially inner side Dri and forms at least partof the inner surface in an outer region on the radially outer side Drofrom the inner region. As described in the first embodiment, thevelocity of the gas flowing through the radial passage 26 is graduallyreduced as it flows to the radially outer side Dro. Therefore, in thisembodiment, in order to increase the sound absorption coefficient of thesecond silencer 30B2, the first surface 32 of the second silencer 30B2is disposed to form at least part of the inner surface in the outerregion of the radial passage 26.

The centrifugal compressor according to this embodiment includes thesecond silencer 30B2 disposed so that the first surface 32 forms part ofthe inner surface of the radial passage 26 in addition to the firstsilencer 30B1 disposed so that the first surface 32 forms part of theinner surface of the outlet scroll passage 27, as in the silencer 30Aaccording to the first embodiment. Accordingly, the centrifugalcompressor according to this embodiment can further increase a soundabsorption amount.

In this embodiment, the inner diameter of the acoustic hole 34 of thesecond silencer 30B2 is preferably smaller than that of the acoustichole 34 of the first silencer 30B 1. This is because the second silencer30B2 is disposed in a region in which the flow rate of the gas is fasterthan that in the first silencer 30B1.

Third Embodiment

A centrifugal compressor according to a third embodiment will bedescribed with reference to FIG. 7.

The centrifugal compressor according to this embodiment also includes afirst silencer 30C1 and a second silencer 30C2, as in the centrifugalcompressor according to the second embodiment. Both the first silencer30C1 and the second silencer 30C2 have the same configuration as thoseof the silencers in the first embodiment and the second embodiment. Thatis, each of the first silencer 30C1 and the second silencer 30C2includes the perforated plate 31 in which the plurality of acousticholes 34 are formed and the space forming body 35 which forms theplurality of acoustic spaces S.

The first silencer 30C1 is disposed so that the first surface 32 of thefirst silencer 30C1 forms part of the external inner surface 27 o of theoutlet scroll passage 27, as in the silencer 30A in the firstembodiment. The first silencer 30C1 may be disposed so that the firstsurface 32 of the perforated plate 31 forms part of one of the innersurfaces which define the outlet scroll passage 27, as described in themodified example of the first embodiment.

The second silencer 30C2 is disposed so that the first surface 32 of thesecond silencer 30C2 forms part of the first side inner surface 26 a ofthe radial passage 26. The second silencer 30C2 only needs to bedisposed so that the first surface 32 of the perforated plate 31 formspart of the inner surface of the radial passage 26. The second silencer30C2 may be disposed to form part of the second side inner surface 26 bof the radial passage 26, for example.

Each of the first silencer 30C1 and the second silencer 30C2 has theplurality of acoustic spaces S. The acoustic spaces S have differentvolumes, for example, to change a sound absorption frequency range.Therefore, it can be said that both the first silencer 30C1 and thesecond silencer 30C2 have a silencer for each of the plurality ofacoustic spaces S. Here, the second silencer 30C2 has three acousticspaces S. That is, the second silencer 30C2 includes three silencers30Ca, 30Cb, and 30Cc. The three silencers 30Ca, 30Cb, and 30Cc in thesecond silencer 30C2 are arranged in the radial direction Dr. Among thethree silencers 30Ca, 30Cb, and 30Cc in the second silencer 30C2, thesilencer 30Ca which is closest to the final stage impeller 11 b in theradial direction Dr is referred to as a second short distance silencer30Ca. The silencer 30Cb adjacent to the radially outer side Dro of thesecond short distance silencer 30Ca is referred to as a secondintermediate distance silencer 30Cb. The silencer 30Cc adjacent to theradially outer side Dro of the second intermediate distance silencer30Cb is referred to as a second long distance silencer 30Cc. Therefore,the velocity of the gas flowing through the passage portion which facesthe second short distance silencer 30Ca is faster than that of the gasflowing through the passage portion which faces the second intermediatedistance silencer 30Cb.

In this embodiment, the inner diameters of the acoustic holes 34 of thesecond long distance silencer 30Cc and the second intermediate distancesilencer 30Cb are the same. In this embodiment, the inner diameter ofthe acoustic hole 34 of the second short distance silencer 30Ca issmaller than those of the acoustic holes 34 of the second long distancesilencer 30Cc and the second intermediate distance silencer 30Cb. Thus,even when the velocity of the gas flowing through the passage portionwhich faces the second short distance silencer 30Ca is high, a decreasein the sound absorption coefficient of the second short distancesilencer 30Ca can be curbed.

The inner diameters of the acoustic holes 34 of the second shortdistance silencer 30Ca, the second intermediate distance silencer 30Cb,and the second long distance silencer 30Cc are preferably smaller thanthat of the acoustic hole 34 of the first silencer 30C1. This is becausepassage distances of the second short distance silencer 30Ca, the secondintermediate distance silencer 30Cb, and the second long distancesilencer 30Cc are shorter than that of the first silencer 30C1 and thevelocity of the gas flowing in the passage portion which faces each ofthe second short distance silencer 30Ca, the second intermediatedistance silencer 30Cb, and the second long distance silencer 30Cc isfaster than that of the gas flowing through the passage portion whichfaces the first silencer 30C1. The passage distance is a distance alongthe outlet passage 25 p from the impeller outlet 14 of the final stageimpeller 11 b. In this case, for example, when the inner diameter of theacoustic hole 34 of the first silencer 30C1 is 1.0 mm, the innerdiameter of the acoustic hole 34 of each of the second long distancesilencer 30Cc and the second intermediate distance silencer 30Cb is setto 0.8 mm, and the inner diameter of the acoustic hole 34 of the secondshort distance silencer 30Ca is set to 0.6 mm.

The inner diameter of the acoustic hole 34 of the second intermediatedistance silencer 30Cb may be smaller than that of the acoustic hole 34of the second long distance silencer 30Cc, and the inner diameter of theacoustic hole 34 of the second short distance silencer 30Ca may besmaller than that of the acoustic hole 34 of the second intermediatedistance silencer 30Cb. In this case, for example, when the innerdiameter of the acoustic hole 34 of the first silencer 30C1 is 1.0 mm,the inner diameter of the acoustic hole 34 of the second long distancesilencer 30Cc is set to 0.8 mm, the inner diameter of the acoustic hole34 of the second intermediate distance silencer 30Cb is set to 0.6 mm,and the inner diameter of the acoustic hole 34 of the second shortdistance silencer 30Ca is set to 0.5 mm.

Fourth Embodiment

A centrifugal compressor according to a fourth embodiment will bedescribed with reference to FIG. 8.

The centrifugal compressor according to each of the above-describedembodiments includes a short distance silencer 30Da and a long distancesilencer 30Db as the silencer. Each of the short distance silencer 30Daand the long distance silencer 30Db includes the perforated plate 31 inwhich the plurality of acoustic holes 34 are formed, and the spaceforming body 35 which forms the plurality of acoustic spaces S, as inthe silencer according to each of the above-described embodiments.

The centrifugal compressor according to each of the above-describedembodiments includes the silencer disposed so that the first surface 32of the perforated plate 31 forms part of the inner surface of the outletscroll passage 27. However, the centrifugal compressor according to thisembodiment does not include a silencer disposed so that the firstsurface 32 of the perforated plate 31 forms part of the inner surface ofthe outlet scroll passage 27. Instead, the centrifugal compressoraccording to this embodiment includes a short distance silencer 30Da anda long distance silencer 30Db disposed so that the first surface 32 ofthe perforated plate 31 forms part of the first side inner surface 26 aof the radial passage 26. The short distance silencer 30Da and the longdistance silencer 30Db may be disposed so that the first surface 32 ofthe perforated plate 31 forms part of the inner surface of the radialpassage 26. The short distance silencer 30Da and the long distancesilencer 30Db may be disposed to form part of the second side innersurface 26 b of the radial passage 26, for example.

The short distance silencer 30Da is disposed on the radially inner sideDri from the long distance silencer 30Db. Thus, the passage distance ofthe short distance silencer 30Da is shorter than that of the longdistance silencer 30Db. In this embodiment, the inner diameter of theacoustic hole 34 of the short distance silencer 30Da is smaller thanthat of the acoustic hole 34 of the long distance silencer 30Db.Specifically, for example, the inner diameter of the acoustic hole 34 ofthe long distance silencer 30Db is 0.8 mm, and the inner diameter of theacoustic hole 34 of the short distance silencer 30Da is 0.5 mm.

Accordingly, in this embodiment, even when the velocity of the gasflowing through the passage portion which faces the short distancesilencer 30Da is higher than that of the gas flowing through the passageportion which faces the long distance silencer 30Db, a decrease in thesound absorption coefficient of the short distance silencer 30Da can becurbed. Since the inner diameter of the acoustic holes 34 of each of thesilencers 30Da and 30Db is selected in consideration of the velocity ofthe gas flowing through the passage portion which faces each of thesilencers 30Da and 30Db, the decrease in the sound absorptioncoefficient can be curbed only with the silencers 30Da and 30Db in whichthe first surface 32 of the perforated plate 31 forms part of the innersurface of the radial passage 26.

Fifth Embodiment

A centrifugal compressor according to a fifth embodiment will bedescribed with reference to FIGS. 9 and 10.

The centrifugal compressor according to this embodiment is a modifiedexample of the centrifugal compressor according to the first embodiment.The centrifugal compressor according to this embodiment is differentfrom the centrifugal compressor according to the first embodiment onlyin the configuration of the silencer, and the other configurations arethe same as those of the centrifugal compressor according to the firstembodiment.

A silencer 30E according to this embodiment includes a perforated plate31 e in which a plurality of acoustic holes 34 are formed, and a spaceforming body 35 e which forms a plurality of acoustic spaces S, as inthe silencer 30A according to the first embodiment. The silencer 30E inthis embodiment is disposed so that the first surface 32 of theperforated plate 31 e forms part of the inner surface of the outletscroll passage 27, as in the silencer 30A in the first embodiment.

The space forming body 35 e of the silencer 30E in this embodimentincludes an outer frame 36 e which forms a space between the outer frame36 e and the perforated plate 31 e on the second surface 33 side of theperforated plate 31 e, and a partition wall 39 e which partitions thespace into a plurality of acoustic spaces S, as in the space formingbody 35 of the silencer 30A in the first embodiment. The outer frame 36e has a bottom wall 37 and a side peripheral wall 38. The bottom wall 37faces the perforated plate 31 e with an interval therebetween. The sideperipheral wall 38 is located between the perforated plate 31 e and thebottom wall 37 and defines a side periphery of the above-describedspace. The partition wall 39 e is integrally formed with the sideperipheral wall 38 of the outer frame 36 e. Among the above-describedportions of the space forming body 35 e, the bottom wall 37 is formed ofthe same material as that of the outlet casing 25 and is integrallyformed with the outlet casing 25. In other words, the bottom wall 37 isformed by part of the outlet casing 25. Among each of the portions ofthe space forming body 35 e, a portion excluding the bottom wall 37, andthe perforated plate 31 e are formed of a material having a largerlinear expansion coefficient than that of the outlet casing 25.

A recessed portion 28 is formed in the outlet casing 25. As shown inFIG. 10, the recessed portion 28 is formed to be recessed from the innersurface of the outlet scroll passage 27 toward the outer side of theoutlet casing 25. A portion of the recessed portion 28 which forms abottom surface forms the bottom wall 37 of the space forming body 35 e.In this embodiment, a portion of the silencer 30E excluding the bottomwall 37 is fitted into the recessed portion 28. However, the portion ofthe silencer 30E excluding the bottom wall 37 is not fixed into therecessed portion 28.

As shown in FIG. 10, when the centrifugal compressor is stopped and theperforated plate 31 e and the outlet casing 25 have the sametemperature, that is, when the centrifugal compressor is sufficientlycooled, there is a gap between the outer frame 36 of the space formingbody 35 e and an inner surface of the recessed portion 28. At this time,there are also gaps between the perforated plate 31 e and the innersurface of the recessed portion 28 and between the perforated plate 31 eand the outer frame 36 of the space forming body 35 e. However, when thecentrifugal compressor is driven, and the temperature of the perforatedplate 31 e and the temperature of the outlet casing 25 are increased,and the perforated plate 31 e and the outlet casing 25 havesubstantially the same temperature, that is, when the centrifugalcompressor is warmed, there is no gap between the outer frame 36 of thespace forming body 35 e and the inner surface of the recessed portion28, as shown in FIG. 9. Further, there is also no gap between theperforated plate 31 e and the inner surface of the recessed portion 28or between the perforated plate 31 e and the outer frame 36 of the spaceforming body 35 e. Accordingly, there is no gap between a plurality ofparts constituting the silencer 30E during the operation of thecentrifugal compressor, and the silencer 30E can exhibit a soundabsorption function.

When the gas compressed by the centrifugal compressor includes acomponent which becomes liquid at room temperature, and the centrifugalcompressor is cooled, and the centrifugal compressor and the gas thereinreach room temperature, some of the gas changes to liquid. The liquidmay remains in the acoustic space S of the silencer 30E. For example, ina staring process of the centrifugal compressor, all the liquid in theacoustic space S may not be vaporized, and some of the liquid may remainas the liquid. In this case, a problem that the silencer 30E cannotabsorb the sound in a target frequency range occurs.

As described above, in the silencer 30E of this embodiment, when thecentrifugal compressor is cooled, the gap is formed between theplurality of parts constituting the silencer 30E. Therefore, even whenthe liquid remains in the acoustic space S, the silencer 30E candischarge the liquid from the gap. Thus, in this embodiment, the problemthat the silencer 30E cannot absorb the sound in the target frequencyrange can be avoided, and thus reduction of the sound absorption effectcan be curbed.

As described above, in the silencer 30E of this embodiment, when thecentrifugal compressor is cooled, the gap is formed between theplurality of parts constituting the silencer 30E. Thus, among the partsconstituting the silencer 30E, those other than the parts integrallyformed with the outlet casing 25 can be easily installed in andseparated from the outlet casing 25.

Sixth Embodiment

A centrifugal compressor according to a sixth embodiment will bedescribed with reference to FIG. 11.

The centrifugal compressor according to this embodiment is a modifiedexample of the centrifugal compressor according to the fifth embodiment.The centrifugal compressor according to this embodiment is differentfrom the centrifugal compressor according to the fifth embodiment onlyin the configuration of the silencer, and the other configurations arethe same as those of the centrifugal compressor according to the fifthembodiment.

A silencer 30F in this embodiment also includes a perforated plate 31 fin which a plurality of acoustic holes 34 are formed, and a spaceforming body 35 f which forms a plurality of acoustic spaces S, as inthe silencers in the first and fifth embodiments. The silencer 30F inthis embodiment is also disposed so that the first surface 32 of theperforated plate 31 f forms part of the inner surface of the outletscroll passage 27, as in the silencers in the first embodiment and thefifth embodiment.

The space forming body 35 f of the silencer 30F in this embodiment isformed of the same material as that of the outlet casing 25 and isintegrally formed with the outlet casing 25. In other words, the spaceforming body 35 f is formed by part of the outlet casing 25. Theperforated plate 31 f of the silencer 30F is formed of a material havinga larger linear expansion coefficient than that of the outlet casing 25.The perforated plate 31 f is not fixed to the space forming body 35 f.

In this embodiment, when the centrifugal compressor is stopped and theperforated plate 31 f and the outlet casing 25 have the sametemperature, that is, when the centrifugal compressor is sufficientlycooled, there is a gap between the perforated plate 31 f and the spaceforming body 35 f formed integrally with the outlet casing 25. When (a)the centrifugal compressor is driven. (b) the temperature of theperforated plate 31 f and the temperature of the outlet casing 25 areincreased, and (c) the perforated plate 31 f and the outlet casing 25have substantially the same temperature (i.e., when the centrifugalcompressor is warmed), there is no gap between the perforated plate 31 fand the space forming body 35 f formed integrally with the outlet casing25. Thus, there is no gap between the plurality of parts constitutingthe silencer 30F during the operation of the centrifugal compressor, andthe silencer 30F can exhibit the sound absorption function.

In the silencer 30F in this embodiment, the gap is formed between theplurality of parts constituting the silencer 30F when the centrifugalcompressor is cooled, as in the silencer 30E in the fifth embodiment.Thus, the silencer 30F can discharge a liquid from the gap even when theliquid remains in the acoustic space S. Therefore, also in thisembodiment, the problem that the silencer 30F cannot absorb the sound inthe target frequency range can be avoided, and the reduction of thesound absorption effect can be curbed.

Also in the silencer 30F in this embodiment, when the centrifugalcompressor is cooled, the gap is formed between the plurality of partsconstituting the silencer 30F. Thus, among the parts constituting thesilencer 30F, those excluding the parts integrally formed with theoutlet casing 25 can be easily installed in and separated from theoutlet casing 25.

The silencers in the fifth embodiment and the sixth embodiment aremodified examples of the silencer 30A in the first embodiment. However,the silencers in the first modified example and the second modifiedexample of the first embodiment, and the second to fourth embodimentsmay be modified in the same manner as that in the silencer in the fifthor sixth embodiment.

Seventh Embodiment

A centrifugal compressor according to a seventh embodiment will bedescribed with reference to FIGS. 12 and 13.

The centrifugal compressor according to this embodiment also includes asilencer 30G as in each of the above-described embodiments and modifiedexamples. As shown in FIG. 12, the silencer 30G according to thisembodiment also includes the perforated plate 31 in which the pluralityof acoustic holes 34 are formed, and the space forming body 35 whichforms the plurality of acoustic spaces S, as in each of theabove-described embodiments and modified examples. The silencer 30G inthis embodiment is disposed so that the first surface 32 of theperforated plate 31 forms part of the inner surface of the radialpassage 26.

The silencer 30G in this embodiment further includes a plurality ofdrain holes 40, a plurality of drain pipes 43, and a plurality of valves44. The drain hole 40 is provided for each of the plurality of acousticspaces S. The drain pipe 43 is provided for each of the plurality ofdrain holes 40. The valve 44 is provided for each of the plurality ofdrain pipes 43. The drain holes 40 include a hole formed in the spaceforming body 35 and a hole formed in the casing 23 (20). The hole formedin the space forming body 35 and the hole formed in the casing 23 (20)are connected to each other. The drain hole 40 has a first opening 41and a second opening 42. The first opening 41 is open in an innersurface of the space forming body 35 which faces the acoustic space S.The second opening 42 is open in an outer surface of a lower portion ofthe casing 23 (20). The first opening 41 is open at a lower portion ofthe inner surface of the space forming body 35 which faces the acousticspace S. Specifically, as shown in FIG. 13, for example, a position ofthe lowermost end 41 x of the first opening 41 is a position at adistance d2 within an inner diameter d1 of the first opening 41 from thelowermost end Sx of the inner surface of the space forming body 35 whichfaces the acoustic space S.

The drain pipe 43 is connected to the second opening 42 of the drainhole 40 as shown in FIG. 12. The valve 44 is provided in the drain pipe43.

Here, as shown in FIG. 13, a portion of the space forming body 35 whichincludes the first opening 41 that opens in the inner surface facing theacoustic space S and which maintains the inner diameter of the firstopening 41 from the inner surface including the first opening 41 in anextending direction of the drain hole 40 is referred to as a firstopening portion 41 a. An inner diameter of the first opening portion 41a is smaller than that of the other portion of the drain hole 40. Inother words, the inner diameter of the drain hole 40 excluding the firstopening portion 41 a is larger than that of the first opening portion 41a. An opening area of the first opening 41 is 20% or less of the totalopening area of all the acoustic holes 34 in a portion of the perforatedplate 31 which defines the acoustic space S in which the first opening41 is open, in other words, all the acoustic holes 34 facing theacoustic space S in which the first opening 41 is open. The opening areaof the first opening 41 is preferably 10% or less of the total openingarea of all the acoustic holes 34. This is to minimize a decrease in thesound absorption effect due to the acoustic space S even when theacoustic space S and the drain hole 40 communicate with each other. Thereason why the inner diameter of the drain hole 40 excluding the firstopening 41 is larger than that of the first opening 41 is to allow theliquid which has passed through the first opening 41 to be easilydischarged outside the casing 20.

A length t (m) of the first opening portion 41 a in the extendingdirection of the drain hole 40 is preferably a value represented by thefollowing Equation (1).

t≤c/fmax/2  (1)

In Equation (1), fmax (Hz) indicates an upper limit of the soundabsorption frequency range by the acoustic space S. and c (m/s)indicates the velocity of sound.

As described above, since the silencer 30G of this embodiment has thedrain hole 40, the liquid remaining in the acoustic space S can bedischarged outside the acoustic space S. Thus, in this embodiment, theproblem in that the silencer 30G cannot absorb the sound in the targetfrequency range can be avoided, and the reduction of the soundabsorption effect can be curbed. When the liquid remaining in theacoustic space S is discharged, the valve 44 is opened. However, whenthe liquid remaining in the acoustic space S is not discharged, thevalve 44 is closed to curb the pressure leak of the gas in the passage.

In this embodiment, the second opening 42 of the drain hole 40 for eachof the plurality of acoustic spaces S is open in an outer surface of thecasing 20. However, for example, when there are three acoustic spaces S,only the second opening 42 of the drain hole 40 of the first acousticspace S may be open in the outer surface of the casing 20. In this case,the second opening 42 of the drain hole 40 in the second acoustic spaceS is open in the first acoustic space S, and the second opening 42 ofthe drain hole 40 in the third acoustic space S is open in the firstacoustic space S or the second acoustic space S. Even in this case, theliquid remaining in the second acoustic space S and the third acousticspace S flows into the first acoustic space S and is then dischargedfrom the first acoustic space S to the outside of the casing 20 throughthe drain hole 40.

Eighth Embodiment

A centrifugal compressor according to an eighth embodiment % ill bedescribed with reference to FIG. 14.

The centrifugal compressor according to this embodiment is a modifiedexample of the centrifugal compressor according to the seventhembodiment. A silencer 30H in this embodiment also includes theperforated plate 31, the space forming body 35, the drain hole 40, thedrain pipe, and the valve, as in the silencer 30G in the seventhembodiment. The perforated plate 31 has the plurality of acoustic holes34 formed therein. The space forming body 35 forms the plurality ofacoustic spaces S. The drain hole 40 is provided for each of theplurality of acoustic spaces S. The drain pipe is provided for each ofthe plurality of drain holes 40. The valve is provided for each of theplurality of drain pipes. FIG. 14 shows only part of the perforatedplate 31, the space forming body 35, and the drain hole 40 among theparts of the silencer 30H and does not show other elements to clarifythe characteristics of the silencer 30H of this embodiment. However, theother elements are the same as the corresponding elements shown in FIG.12 showing the centrifugal compressor according to the seventhembodiment.

As in the silencer 30G of the seventh embodiment, the silencer 30H inthis embodiment is also disposed so that the first surface 32 of theperforated plate 31 forms part of the inner surface of the radialpassage 26. The drain hole 40 in this embodiment has a first opening 41h and a second opening, as in the drain hole 40 in the seventhembodiment. The first opening 41 h is open in the inner surface of thespace forming body 35 which faces the acoustic space S. The secondopening is open in the outer surface of the lower portion of the casing20. The first opening 41 h of the drain hole 40 in this embodiment isbasically not limited in the opening area, as in the first opening 41 ofthe drain hole 40 in the seventh embodiment. Therefore, the opening areaof the first opening 41 h of the drain hole 40 in this embodiment may belarger than that of the first opening 41 of the drain hole 40 in theseventh embodiment.

The silencer 30H in this embodiment further includes a moistureabsorbing member 45 disposed in the drain hole 40. The moistureabsorbing member 45 includes particles of a plurality of moistureabsorbents 46, and an adhesive which connects the particles of theplurality of moisture absorbents 46. The moisture absorbent 46 is asolid which absorbs moisture, or a material which can be handled as asolid while moisture is absorbed. Specifically, the moisture absorbent46 is silica gel, calcium chloride, calcium oxide or the like. An amountof the moisture absorbent 46 in the moisture absorbing member 45 isdetermined on the basis of an amount of liquid assumed to remain in theacoustic space S, a moisture absorption capacity of the moistureabsorbent 46, an environmental change such as a temperature change and apressure change in the acoustic space S, and the like.

The moisture absorbing member 45 is disposed at a position in the drainhole 40 including the first opening 41 h or a position close to thefirst opening 41 h. In other words, the moisture absorbing member 45 isdisposed at a position in the drain hole 40 substantially including thefirst opening 41 h. In a region in the drain hole 40 in which themoisture absorbing member 45 is disposed in the extending direction ofthe drain hole 40, the moisture absorbing member 45 is provided so thatthe drain hole 40 is closed. That is, an area of the drain hole 40 in adirection perpendicular to the extending direction of the drain hole 40is substantially equal to a cross-sectional area of the moistureabsorbing member 45 in the direction perpendicular to the extendingdirection of the drain hole 40. Therefore, in this embodiment, even whenthe acoustic space S and the drain hole 40 communicate with each other,the decrease in the sound absorption effect due to the acoustic space Scan be minimized.

Since the silencer 30H of this embodiment also has the drain hole 40 asdescribed above, the liquid remaining in the acoustic space S can bedischarged outside the acoustic space S. Specifically, the liquidcomponent in the acoustic space S is adsorbed by the moisture absorbingmember 45 in the drain hole 40. When the liquid component adsorbed bythe moisture absorbing member 45 is discharged, the valve 44 is opened.As a result, the liquid component adsorbed by the moisture absorbingmember 45 is discharged outside the casing 20 through the drain hole 40and the drain pipe 43. When the liquid component adsorbed by themoisture absorbing member 45 is not discharged, the valve 44 is closedto curb the pressure leak of the gas in the passage.

Also in this embodiment, the liquid remaining in the acoustic space Scan be discharged outside the acoustic space S. Therefore, the problemthat the silencer 30H cannot absorb the sound in the target frequencyrange can be avoided, and the reduction of the sound absorption effectcan be curbed. Furthermore, in this embodiment, since the moistureabsorbing member 45 is provided in the drain hole 40 to close the drainhole 40, the reduction of the sound absorption effect due to thecommunication between the acoustic space S and the drain hole 40 isfurther curbed.

The second opening of the drain hole 40 for each of the plurality ofacoustic spaces S in this embodiment is open in the outer surface of thecasing 20, as in the seventh embodiment. However, for example, whenthere are three acoustic spaces S, only the second opening of the drainhole 40 of the first acoustic space S may be open in the outer surfaceof the casing 20. In this case, the second opening of the drain hole 40of the second acoustic space S is open in the first acoustic space S,and the second opening of the drain hole 40 of the third acoustic spaceS is open in the first acoustic space S or the second acoustic space S.The moisture absorbing member 45 is disposed in each of the drain holes40, as in this embodiments. Also in this case, the liquid remaining inthe second acoustic space S and the third acoustic space S flows intothe first acoustic space S and is then discharged from the firstacoustic space S through the drain hole 40 to the outside of the casing20.

Ninth Embodiment

A centrifugal compressor according to a ninth embodiment will bedescribed with reference to FIG. 15.

The centrifugal compressor according to this embodiment is a modifiedexample of the centrifugal compressor according to the eighthembodiment. A silencer 30I in this embodiment also includes theperforated plate 31, the space forming body 35, the drain hole 40, andthe moisture absorbing member 45, as in the silencer 30H in the eighthembodiment. A plurality of acoustic holes 34 are formed in theperforated plate 31. The space forming body 35 forms a plurality ofacoustic spaces S. The drain hole 40 is formed for each of the pluralityof acoustic spaces S. The moisture absorbing member 45 is provided foreach of the plurality of drain holes 40. However, the silencer 30Iaccording to this embodiment does not include the drain pipe 43 providedfor each of the plurality of drain holes 40 and the valve 44 providedfor each of the plurality of drain pipes 43 in the silencers 30G and 30Haccording to the seventh and eighth embodiments

As in the silencer 30G of the eighth embodiment, the silencer 30I inthis embodiment is also disposed so that the first surface 32 of theperforated plate 31 forms part of the inner surface of the radialpassage 26. Unlike the drain hole 40 in the seventh embodiment and theeighth embodiment, the drain hole 40 i of this embodiment is formed inthe perforated plate 31. The drain hole 40 i has a first opening 41 iand a second opening 42 i. The first opening 41 i is open in the firstsurface 32 of the perforated plate 31. The second opening 42 i is openin the second surface 33 of the perforated plate 31. The second opening42 is open at a lower portion of the perforated plate 31. Specifically,for example, a position of the lowermost end of the second opening 42 isa position at a distance within the inner diameter of the second opening42 from the lowermost end in the acoustic space S.

The moisture absorbing member 45 is the same as the moisture absorbingmember 45 of the eighth embodiment. The moisture absorbing member 45 isdisposed in the drain hole 40 i to close the drain hole 40 i. Thus, evenwhen the acoustic space S and the drain hole 40 i communicate with eachother, the decrease in the sound absorption effect due to the acousticspace S can be minimized.

Since the silencer 30I of this embodiment also has the drain hole 40 ias described above, the liquid remaining in the acoustic space S can bedischarged outside the acoustic space S. Specifically, the liquidcomponent in the acoustic space S is adsorbed by the moisture absorbingmember 45 in the drain hole 40 i. The liquid component adsorbed by themoisture absorbing member 45 is discharged into the radial passage 26.Therefore, in this embodiment, the problem that the silencer 30I cannotabsorb the sound in the target frequency range can be avoided, and thereduction of the sound absorption effect can be curbed. In thisembodiment, since the moisture absorbing member 45 is provided in thedrain hole 40 i to close the drain hole 40 i, the reduction of the soundabsorption effect can be further curbed as compared with the seventhembodiment. In the silencer 30I according to this embodiment, the drainpipe 43 and the valve 44 in the silencers 30G and 30H in the seventh andeighth embodiments are unnecessary, and the drain hole 40 i only needsto be formed in the perforated plate 31. As a result, the silencer 30Ican be simplified in structure, and manufacturing cost can be reduced.

The drain discharge mechanism of any one of the silencers 30G, 30H, and30I in the above-described seventh to ninth embodiments may be appliedto the silencer in each of the first to fourth embodiments and each ofthe modified examples of the first embodiment. The drain dischargemechanism of any one of the silencers 30G, 30H, and 30I in theabove-described seventh to ninth embodiments may be applied to asilencer in which the first surface 32 of the perforated plate 31 isdisposed to form part of the inner surface of the intermediate passage23 p (shown in FIG. 1).

Further, although the centrifugal compressors in the above-describedembodiments and modified examples are all multistage centrifugalcompressors, the centrifugal compressor according to the presentinvention is not limited to the multistage centrifugal compressors. Thatis, the centrifugal compressor according to the present invention may bea single-stage centrifugal compressor having one impeller.

What is claimed is:
 1. A centrifugal compressor comprising: one or moreimpellers configured to rotate about an axis and to send a gas to aradially outer side of the axis; a casing configured to cover the one ormore impellers and to guide the gas from an inlet to an inside todischarge the gas from an outlet and having an outlet scroll passagewhich extends in a circumferential direction of the axis andcommunicates with the outlet; and a silencer, wherein the silencerincludes a perforated plate in which a first surface and a secondsurface opposite to the first surface and a plurality of acoustic holespenetrating from the first surface to the second surface are formed, anda space forming body configured to form an acoustic space connected tothe plurality of acoustic holes on a second surface side of theperforated plate, and the first surface of the silencer forms part of aninner surface of the casing which defines the outlet scroll passage. 2.The centrifugal compressor according to claim 1, wherein the firstsurface of the silencer forms only part of the inner surface of theoutlet scroll passage.
 3. The centrifugal compressor according to claim1, wherein: a first silencer and a second silencer are provided as thesilencers, the casing has a radial passage which guides the gas flowingout from a final stage impeller of the one or more impellers to theradially outer side and communicates with the outlet scroll passage, thefirst surface of the first silencer forms part of the inner surface ofthe outlet scroll passage, and the first surface of the second silencerforms part of an inner surface which defines the radial passage.
 4. Thecentrifugal compressor according to claim 3, wherein the first surfaceof the second silencer does not form an inner surface in an inner regionincluding an end on a radially inner side of the axis but forms at leastpart of the inner surface in an outer region on the radially outer sidefrom the inner region in the radial passage.
 5. The centrifugalcompressor according to claim 3, wherein: a short distance silencer anda long distance silencer are provided as the second silencers, the shortdistance silencer is disposed at a position closer to the final stageimpeller than the long distance silencer in a radial direction of theaxis, and inner diameters of the plurality of acoustic holes of theshort distance silencer are smaller than those of the plurality ofacoustic holes of the long distance silencer.
 6. The centrifugalcompressor according to claim 1, wherein at least part of the spaceforming body of the silencer is formed of the same material as that ofthe casing and integrally formed with the casing.
 7. The centrifugalcompressor according to claim 6, wherein the perforated plate is formedof a material having a linear expansion coefficient larger than that ofthe casing.
 8. The centrifugal compressor according to claim 1, whereinthe silencer has a drain hole which discharges a liquid in the acousticspace outside of the acoustic space.
 9. The centrifugal compressoraccording to claim 8, wherein: the drain hole has a first opening whichopens in the first surface of the perforated plate, and a second openingwhich opens in the second surface of the perforated plate, and amoisture absorbent is disposed in the drain hole.
 10. The centrifugalcompressor according to claim 1, wherein inner diameters of theplurality of acoustic holes are 0.1 to 3.0 mm.
 11. The centrifugalcompressor according to claim 10, wherein inner diameters of theplurality of acoustic holes are 0.2 to 1.0 mm.
 12. A centrifugalcompressor comprising: one or more impellers configured to rotate aboutan axis and to send a gas to a radially outer side of the axis; a casingconfigured to cover the one or more impellers and to guide the gas froman inlet to an inside to discharge the gas from an outlet and having anoutlet passage which guides the gas flowing out from a final stageimpeller of the one or more impellers to the outlet; and a shortdistance silencer and a long distance silencer as silencers, whereineach of the short distance silencer and the long distance silencerincludes a perforated plate in which a first surface and a secondsurface opposite to the first surface and a plurality of acoustic holespenetrating from the first surface to the second surface are formed, anda space forming body configured to form an acoustic space connected tothe plurality of acoustic holes on a second surface side of theperforated plate, both the first surface of the short distance silencerand the first surface of the long distance silencer form part of aninner surface of the casing which defines the outlet passage, a distancealong the outlet passage from the final stage impeller to the shortdistance silencer is shorter than that along the outlet passage from thefinal stage impeller to the long distance silencer, and inner diametersof the plurality of acoustic holes of the short distance silencer aresmaller than those of the plurality of acoustic holes of the longdistance silencer.
 13. The centrifugal compressor according to claim 12,wherein at least part of the space forming body of the silencer isformed of the same material as that of the casing and integrally formedwith the casing.
 14. The centrifugal compressor according to claim 13,wherein the perforated plate is formed of a material having a linearexpansion coefficient larger than that of the casing.
 15. Thecentrifugal compressor according to claim 12, wherein the silencer has adrain hole which discharges a liquid in the acoustic space outside ofthe acoustic space.
 16. The centrifugal compressor according to claim15, wherein the drain hole has a first opening which opens in an innersurface of the space forming body which faces the acoustic space, and asecond opening which opens in an outer surface of the casing.
 17. Thecentrifugal compressor according to claim 15, wherein: the drain holehas a first opening which opens in the first surface of the perforatedplate, and a second opening which opens in the second surface of theperforated plate, and a moisture absorbent is disposed in the drainhole.
 18. The centrifugal compressor according to claim 12, whereininner diameters of the plurality of acoustic holes are 0.1 to 3.0 mm.19. The centrifugal compressor according to claim 18, wherein innerdiameters of the plurality of acoustic holes are 0.2 to 1.0 mm.
 20. Acentrifugal compressor comprising: one or more impellers configured torotate about an axis and to send a gas to a radially outer side of theaxis; a casing configured to cover the one or more impellers and toguide the gas from an inlet to an inside to discharge the gas from anoutlet and having a passage which guides the gas flowing in from theinlet to the outlet through the one or more impellers; and a silencer,wherein the silencer includes a perforated plate in which a firstsurface and a second surface opposite to the first surface and aplurality of acoustic holes penetrating from the first surface to thesecond surface are formed, a space forming body configured to form anacoustic space connected to the plurality of acoustic holes on a secondsurface side of the perforated plate, and a drain hole configured todischarge a liquid in the acoustic space to an outside of the acousticspace, and the first surface of the silencer forms part of an innersurface of the passage.
 21. The centrifugal compressor according toclaim 20, wherein the drain hole has a first opening which opens in aninner surface of the space forming body which faces the acoustic space,and a second opening which opens in an outer surface of the casing. 22.The centrifugal compressor according to claim 21, wherein an openingarea of the first opening is 20% or less of a total opening area of allthe acoustic holes in a portion of the perforated plate which definesone acoustic space.
 23. The centrifugal compressor according to claim21, wherein the silencer has a valve capable of partitioning a space inthe drain hole from an outer space of the casing.
 24. The centrifugalcompressor according to claim 21, wherein a moisture absorbent isdisposed in the drain hole.
 25. The centrifugal compressor according toclaim 20, wherein: the drain hole has a first opening which opens in thefirst surface of the perforated plate, and a second opening which opensin the second surface of the perforated plate, and a moisture absorbentis disposed in the drain hole.
 26. The centrifugal compressor accordingto claim 20, wherein inner diameters of the plurality of acoustic holesare 0.1 to 3.0 mm.
 27. The centrifugal compressor according to claim 26,wherein inner diameters of the plurality of acoustic holes are 0.2 to1.0 mm.
 28. A centrifugal compressor comprising: one or more impellersconfigured to rotate about an axis and to send a gas to a radially outerside of the axis; a casing configured to cover the one or more impellersand to guide the gas from an inlet to an inside to discharge the gasfrom an outlet; and a silencer, wherein the casing has a passageconfigured to guide the gas flowing in from the inlet to the outletthrough the one or more impellers, the silencer includes a perforatedplate in which a first surface and a second surface opposite to thefirst surface and a plurality of acoustic holes penetrating from thefirst surface to the second surface are formed, and a space forming bodyconfigured to form an acoustic space connected to the plurality ofacoustic holes on a second surface side of the perforated plate, thefirst surface of the silencer forms part of an inner surface of thecasing which defines the passage, at least part of the space formingbody is formed of the same material as that of the casing and integrallyformed with the casing, and the perforated plate is formed of a materialhaving a linear expansion coefficient larger than that of the casing.29. The centrifugal compressor according to claim 28, wherein innerdiameters of the plurality of acoustic holes are 0.1 mm or more to 3.0mm or less.
 30. The centrifugal compressor according to claim 29,wherein inner diameters of the plurality of acoustic holes are 0.2 mm ormore to 1.0 mm or less.